Summary
Previous studies had shown that when gel-filtered or washed human platelets were incubated
at pH 5.3, the cells secreted their granule-stored materials suggesting that low pH
can act as a platelet activator. We determined here whether the effects of low pH
on platelet protein phosphorylation and on platelet lipid metabolism were consistent
with this view. When washed human platelets were incubated for 20 min at pH 5.3 and
electrophoresed on SDS-PAGE, there was a great increase in 32P-label in the 20,000 and 47,000 dalton protein bands. There was also an increase
in the labeling of phosphatidic acid and a small decrease in phosphatidyl inositol.
When the platelets were returned to pH 7.6, the 32P labeling of the 20,000 and 47,000 dalton bands was greatly reduced, and that of
phosphatidic acid reduced to the control value, while the labeling of phosphatidyl
inositol was increased above control. Incubation at pH 5.3 for 60 min gave the same
pattern, but return to pH 7.6 resulted in only partial reversal of labeling of the
two protein bands and little decrease in the label associated with phosphatidic acid,
but the radioactivity in phosphatidyl inositol was greatly increased. The changes
in the 32P-labeling of phospholipids and proteins after incubation of platelets at pH 5.3 may
reflect an increase in cytoplasmic Ca++ resulting from leakage of Ca++ from intracellular storage sites, a process which becomes irreversible after longer
time exposure to the low pH. The activation of human platelets at pH 5.3 is a slow
process which may not be directly comparable to the fast events in the normal stimulation-response
coupling, but which is accompanied by changes common to platelet activation. Most
interesting in this respect is that a return to physiological conditions will reverse
these events, although prolonged exposure will make them irreversible. These studies
may therefore present a way to study the boundary between reversible and irreversible
processes in platelets.
Key words
Phosphorus labeling - Platelet protein - Phospholipids - Platelet activation - Reversibility
- pH 5.3
